In the United States and many other countries, large numbers of swimming pools are in use and currently are being built. While some of these pools are "public" pools where use is restricted to time periods when lifeguards are present, many such pools are located in the yards of private homes. These private pools, in particular, present a constant danger to non-swimmers, particularly small children. Unprotected swimming pools are a significant cause of the loss of life through drowning of small children. The problem is sufficiently severe that many local authorities now require protective fencing around such pools and between the pool and any home with which it is associated.
Even though protective fencing currently is legislated in most areas for new pool construction, many older swimming pools exist which do not have protective fencing. Even with such fencing, however, small children frequently enter the pool area through an unlatched gate, or find some other way to surmount the fence to reach the pool. Toddlers in particular are difficult to protect from the dangers of swimming pools since they move quickly, often ignore warnings and are extremely curious.
Various devices and systems have been developed for setting off an alarm when a person or some other object enters a pool during the time the alarm is activated. Systems of this type, which do not include any device or portion worn by a person, are disclosed in the patents to Wolf U.S. Pat. No. 4,510,487; Woolley U.S. Pat. No. 4,571,579; Baker U.S. Pat. No. 4,604,610; and Dunegan U.S. Pat. No. 4,747,083. The devices shown in the Wolf and Woolley patents are activated by wave action in the pool. A sensor detects the wave action and the sensed wave action is converted electrically into a signal which is used to trigger an audible alarm.
The device disclosed in the Baker U.S. Pat. No. 4,604,610 employs a hydrophone, located beneath the surface of the pool and coupled with a sensor, to respond to vertical wave motion of the pool water beneath the surface impacting the hydrophone transducer. The hydrophone is electrically connected with an amplifier circuit for triggering an alarm in response to wave motion which exceeds a preestablished threshold.
A different approach is taken in the Dunegan U.S. Pat. No. 4,747,085. In Dunegan, a transmitter is mounted below the surface of a swimming pool to continuously transmit ultrasonic sound waves through the body of water. An underwater receiver is positioned to detect the sound waves; and the receiver produces an electrical signal which is monitored. Whenever movement of a person in the pool occurs, the received ultrasonic sound waves are altered, causing an alteration of the electrical signal produced by the receiver. This altered signal then is used to actuate an alarm.
A disadvantage which is common with all of the devices of the patents discussed above is that the alarm is triggered any time any person enters the water of the pool or swims in the pool. Consequently, when normal use of the pool is desired, the alarms must be de-activated or turned off. Frequently, the pool owner or user will forget to re-activate the alarm or turn it back on after use of the pool has taken place. Even during periods of normal pool use, for example when a family is actively engaged in water activities, in and out of the pool, small children frequently and readily can slip unnoticed into the water. If the alarm is turned off, such a child may not be detected in time to save the child's life. Even if the alarm is turned on, small children frequently slip into the water with little or no wave motion; so that to be effective in protecting against the drowning of such children, the alarm devices must be extremely sensitive. When the sensitivity of such alarms is increased, false alarm conditions frequently are produced. If a sufficient number of false alarm operations take place, most persons disconnect the alarm system; and its intended purpose is thereby defeated.
Attempts to overcome the disadvantages of the patents discussed above have been made by attaching some type of water-activated switch or alarm directly to a person to be protected. Four patents which are directed to this general type of pool or water safety alarm are the patents to Antenore U.S. Pat. No. 4,079,364; Moura U.S. Pat. No. 4,549,169; Sackett U.S. Pat. No. 4,620,181; and Boe U.S. Pat. No. 4,714,914. All four of these patents are directed to water-activated switches of some type, which directly operate an alarm or activate a transmitter for transmitting a signal to an alarm whenever a switch is activated. In each case, the transmitter or activating portion is worn by the person being monitored. The manner in which the devices are operated and in which they are activated, differs, but all of them have this basic feature in common.
A somewhat different approach to monitoring the activity of a person in a body of water is disclosed in the patent to Bianco U.S. Pat. No. 3,786,406. The Bianco device includes an underwater sonar transmitter and an underwater sonar receiver in constant communication with one another. The person wearing the transmitter must periodically close a switch to keep it in operation. An alarm is sounded when the time interval between successive closures of the transmitter-activating switch exceeds some pre-established limit. Because of the necessity for constantly closing the transmitter switch, this device is not suitable for protecting small children or non-swimmers who might accidently fall into a body of water. The Bianco device has utility in monitoring the activity of a diver or underwater swimmer.
A system which is perhaps of most utility for protecting nonswimmers, such as babies and toddlers in particular, is disclosed in the patent Lieb U.S. Pat. No. 3,810,146. The system of this device includes a portable ultrasonic (sonar) transmitter which is attached to the clothes of the person to be protected, or otherwise attached to the body of such a person. The transmitter is activated for operation upon immersion in water. An underwater receiver responds to the ultrasonic wave generated by the transmitter to operate an alarm upon receiving the transmitted signal. The nature of the device disclosed in the Lieb patent is such that the alarm can be left activated or turned on at all times, including times when other swimmers are using the pool. Since the frequency of the mechanical wave which is produced by the transmitter is ultrasonic, the danger of "false alarms" also is reduced. A problem with the use of an ultrasonic transmitter/receiver, however, is that ultrasonic waves are "directional"; so there is a danger that an alarm condition could exist without the receiver detecting the condition in sufficient time to actuate the alarm.
It is desirable to provide an improved pool safety alarm system which overcomes the disadvantages of the prior art, which is effective in use, which is not subject to false alarms, and which is capable of providing an indication, in a stand-by mode, that it is functioning properly.